Collapsible revolving door



Feb. 10, 1970 D. LOWE 3,

COLLAPSIBLE REVOLVING DOOR Filed NOV. 21, 1968 6 Shegts-Sheet l r 52 10 118 a E INVENTOR. DENNIS LOWE r'fau Feb. 10, 1970 D. LOWE COLLAPSIBLE REVOLVING DOOR s sheets-sheet 2 Filed Nov. 21, 1968 we w m0 L m N N E D Feb. 10, 1970 D. LOWE 3,495,251

I COLLAPSIBLE REVOLVING DOOR Filed Nov. 21, 1968 6 Sheets-Sheet 5 INVENTOR. DENNIS LOWE Feb; 10, 1970 D. Lows 3,495,251

. OOLLAPSIBLE REVOLVING DOOR Filed Nov. 21, 1968 6 Sheets-Sheet 4 0% gm co m m O Q co ("3 I] 1- I 01 s q 0 w m (D 1: wgvgv g O L m a m (D h cu v O o I a r INVENTOR.

DENNIS LOWE BY Feb. 10, 1970 n. LOWE 3,495,251

COLLAPSIBLE REVOLVING DOOR Filed Nov. 21, 1968 e Sheets-Sheets 118 g 1060 50 k v 1 47 o \m A E X 7 Q \L I \I \l o 0 v a 0 2 1J4 3110." o'-" a 0 I .o0

FIG 8 76 INVENTOR. DENNIS LOWE Feb. 10, 1970 D. LOWE 3,

- COLLAPSIBLE REVOLVING DOOR Filed Nov. 21, 1968 v 6 Sheets-Sheet 6 INVENTOR. DENNIS LOWE United States Patent 3,495,251 COLLAPSIBLE REVOLVING DOOR Dennis Lowe, 58 Pitfield Road, Agincourt, Ontario, Canada Filed Nov. 21, 1968, Ser. No. 777,648 Claims priority, application Canada, Apr. 24, 1968, 018,248 Int. Cl. Ed 15/02 US. CI. 49-44 11 Claims ABSTRACT OF THE DISCLOSURE A mechanism enabling folding of revolving door leaves under panic conditions, having earn housings mounted at the top and bottom of the door central shaft to rotate with the door. Each housing contains a central cam, and four radially extending cam followers (one for each door leaf) each held against the cam by a detent. Pins extend from the ends of the cam followers adjacent the cam, through radial slots in the housing to hold the leaves. Under panic conditions the detents yield, the cam followers and leaves rotate about the pins, and the pins move outwardly in their slots, enabling folding.

This invention relates to a revolving door of the type having door leaves mounted to project radially from a central rotary shaft and normally rotating with the central shaft. More specifically, the invention relates to a revolving door in which the leaves are adapted to fold under abnormal pressure occurring under panic conditions, so as to create an open passage through the doorway. Revolving doors of this type are often referred to as collapsible revolving doors.

Many different types of collapsible revolving doors have been built in the past. However, such prior art doors have generally suffered from at least one and often both of two main defects. Firstly, a costly and complicated mechanism has usually been required to retain the door leaves normally in their uncollapsed or extended position and to guide the leaves to their collapsed position during folding, and secondly, this mechanism has in large part been exposed to view, creating an unsightly appearance for the door.

Accordingly, it is an object of the present invention to provide a relatively simple mechanism for normally retaining the leaves of a collapsible revolving door in an extended position while enabling their folding under panic conditions, such mechanism being of a type that may be substantially entirely concealed, so that the door has a clean and modern appearance, instead of the usually cluttered appearance.

In a typical embodiment, a mechanism according to the invention includes a cam housing having a central cam thereon. One such housing will be mounted on the central door shaft at the top of the door, and one on the central door shaft at the bottom of the door, to rotate with the door. Each housing typically includes four cam followers, one for each door leaf (for a four leaf door), each cam follower extending radially outwardly from the cam and normally being held in position by a detent. Pins extend from the ends of the cam followers adjacent the cam through radial slots in the cam housing to hold the door leaves. Under panic conditions the detents yield. allowing the door leaves and cam followers 3,495,251 Patented Feb. 10, 1970 to rotate about the pins. During rotation the pins move outwardly in their slots, enabling folding.

Further objects and advantages of the invention will appear from the following disclosure, taken together with the accompanying drawings, in which:

FIG. 1 is a perspective view of a typical revolving door according to the invention;

FIG. 2 is a perspective view of an upper cam housing of the door of FIG. 1;

FIG. 3 is a plan view of the cam housing of FIG. 2;

FIG. 4 is a perspective view, partly broken away, of the upper cam housing of FIGS. 2 and 3 with cam followers in position therein, and showing the connections from the cam followers to the door leaves;

FIG. 5 is a plan view of a rubber block shown in FIG. 4;

FIG. 6 is a plan view of a cam follower of FIG. 4;

FIG. 7 is a perspective view of a ball socket of FIG. 4;

FIG. 8 is a sectional view of the door of FIG. 1;

FIG. 9 is a section along lines 9-9 of FIG. 8;

FIGS. 10 to 13 show diagrammatically the movement of the door leaves and cam followers in the upper cam housing during folding of the leaves;

FIG. 14 is a top sectional view showing the manner in which a ball socket fits into a cam housing; and

FIG. 15 is a perspective view of a ball socket for the bottom of the door.

Reference is first made to FIGURE 1, which shows in perspective a typical revolving door 2 according to the present invention. The door 2 includes four leaves 4 mounted to rotate with a central shaft 6 normally hidden from view by a weather barrier 8. The tops of the leaves 4 and central shaft 6 are connected to rotate with an upper cam housing 10 recessed in the ceiling 12, while the bottoms of the leaves 4 and shaft 6 are similarly connected to rotate with a lower cam housing 14 recessed in the floor 16. The cam housings 10, 14 contain the mechanism for controlling the collapse of the leaves, as will be described.

Reference is next made to FIGS. 2 and 3, which show the upper cam housing 10. The lower cam housing 14 and associated parts are identical with the upper cam housing and associated parts except for minor differences to be described, and primed reference numerals in the drawings (FIGS. 1, 8, 15) will be used to indicate parts in the lower portion of the door corresponding to those in the upper portion of the door.

The upper cam housing 1.0 includes a central cam 18 having a central aperture 20 for the shaft 6, a base plate 22 typically cast integrally with the cam .18, and a circumferential outer wall 24 concentric with the aperture 20 and also typically formed during the casting. The cam 18 has a camming profile oriented in a direction parallel to the axis of shaft 6 (i.e. in a vertical direction when the door is assembled), the camming profile including four depressions 26, one associated with each leaf 4, and four lobes 28 separating the depressions.

The base plate 22 includes four slots 30, one for each leaf 4. Each slot 30 extends radially outwardly from a position adjacent a cam profile depression 26 to a position adjacent the outer wall 24.

The outer wall 24 is of generally uniform cross-section but includes four counter-bored apertures 32 to accommodate part of the detent means (to be described) for holding the leaves normally in extended position. The bottom of the outer wall is relieved as shown at 34, over an angle of about 20 degrees on each side of the apertures 32, to accommorate optional handles for manually controlling the detent means, as will also be described.

Finally, the upper cam housing includes a light cover plate 35 (visible only in FIG. 8) to keep out dust and other dirt.

Reference is next made to FIG. 4, which shows the connection of the cam housing 10 to the shaft 6, and the connections from the cam housing to the door leaves 4. As there shown, the upper cam housing 10 contains four cam followers 36, one associated with each door leaf 4. The cam followers 36 are movable inside the housing 10, as will be explained, and are shown in their uncollapsed or extended position in FIG. 4. Each cam follower 36 has a rounded root 38 which normally rests in one of the depressions 26 in the cam profile, and a relatively wide body 40 extending radially outwardly substantially to the circumferential wall 24 of the housing 10. When the cam followers are in this position, each overlies and almost completely covers one of the slots 30 in the base plate 22. (The cam follower bodies 40 are typically about 2 inches in width, while the slots 30 are typically about 1 inch in width.)

Each cam follower includes an aperture 42 in which is fixed a leaf pin 44. The pins 44 are held firmly to the cam followers 36 by silver soldering the top inch of each pin to the cam followers.

Each leaf pin 44 extends through one of the slots 30 and includes a square bottom 45 accommodated in a square socket 46 of a sturdy leaf support arm 47 made of stainless steel or the like. The back of each leaf support arm is split as shown at 48 to form a clamp having apertures to accommodate clamping screws 49. When the screws 49 are tightened, this clamps the pin 44 securely to the leaf support arms 47.

The leaf support arms 47 are elongated (they are typically about 12 inches long) and extend through correspondingly shaped slots 50 near the upper edges of the leaves 4, to provide a firm connection to the leaves. In order the seal any gap between the leaf support arms and the base plate 22 of the cam housing that might allow dirt to enter the cam housing, a neoprene rubber block 52 (see also FIG. encircles the upper part of each leaf support arm and lies adjacent the underside of base plate 22.

The cam followers 36 are hollow and each includes a ball retainer cap 56 (see also FIG. 6) held by set screws 58 to the main body of the cam follower. The ball retainer cap 56 includes rounded inner edges 60 which retain a spherical stainless steel ball 62 (typically one inch in diameter) inside the cam follower 36 with a small portion of the ball (e.g. one-quarter of its diameter) projecting therefrom. A heavy coil spring 64 located in the space inside the cam follower urges the ball outwardly to its extended position.

When the cam followers 36 are in the position shown in FIG. 4, each ball 62 is pressed by its associated spring 64 into a ball socket 66 located in each aperture 32 in the circumferential wall 24. The sockets 66 (shown in detail in FIG. 7) are formed as circular members having front flanges 68 which fit into the enlarged front part of the apertures 32 to prevent sockets 66 from being pushed outwardly by the pressure of spring 64 acting on balls 62.

In the embodiment illustrated in FIG. 4, the sockets 66 are rotatable, by handles 70 attached thereto, to facilitate manual folding of the door. (The feature of making the sockets 66 rotatable is an optional feature, as will be described.) In this arrangement the front faces of the sockets contain grooves 72 which are normally vertically aligned and into which the balls 62 are pressed. (For a 1 inch diameter ball, the groves 72 will typically have a curvature corresponding to a 1 inch diameter and a depth of Ms inch at their deepest point.) The sockets may be rotated by handles 70 to a position in which the 4 grooves 72 extend in a horizontal direction, for a purpose to be described.

Reference is next made to FIG. 8 which shows how the shaft 6, the upper and lower cam housings 10 and 14, and their associated parts are mounted on the floor and ceiling and connected to the leaves 4. The lower cam housing and its mounting will first be described.

As discussed, the lower cam housing 14 is almost identical with the upper cam housing 10. However, the lower cam housing is provided with a thick cover plate 35, the thickness being needed because, in the embodiment shown, the cover plate carries the entire weight of the door. The cover plate 35' rests on a support 73 mounted on a thrust bearing mounting 74 which supports and aligns the bottom of the door. The bearing mounting 74 rests on the top 75 of a casing 76 supported in a well 77 in the floor itself. The square head 78 of a shaft 79 extends downwardly from a square aperture in the central shaft 6, into a speed control mechanism (not shown) in the casing 76. The speed control mechanism limits the speed of rotation of the door, and can be omitted if desired.

The base plate 22' of the lower cam housing contains a lip 80 which extends radially outwardly over a ring 82 set in the floor 16 to prevent dirt from falling through the space between the floor and the side wall 24' of the lower cam housing. A spring biased lubricant impregnated nylon ring 84 rests in a slot in the ring 82 and is biased upwardly against the lip '80 by a rubber (neoprene) spring 86 to maintain a good seal, to prevent dirt from falling into the space mentioned.

The shaft 6 is fixed to the lower cam housing (specifically to the cam 18') by any desired means, such as welding or pinning, and extends upwardly therefrom. The shaft 6 is formed in three sections, namely, a lower section 6a, an upper section 6b separated from the lower section by a gap 88, and an outer pipe 6c encircling inner sections 6a, 6b. The gap 88 is provided to permit telescoping of the upper and lower sections 6a, 6b for installation or removal of the cam housings when the outer pipe 60 is removed. The pipe 6c is welded to upper section 6b and is pinned to lower section 6a (when section 6a is in position) by pins 90.

The upper cam housing 10 is supported near the top of the shaft 6 by connection of cam 18 to the shaft 6. The connection may be by welding or by pins. Once the door leaves 4 are in position, the upper cam housing 10 will also be supported by these leaves. Above the upper cam 18, the shaft 6 rotates in a bearing 94 fixed to heavy structural members 98 in the ceiling 12. The portion of the ceiling 12 normally visible to viewers is indicated in FIG. 8 at 100 and is typically made of sheet aluminum held to structural members 98 by supports 102.

After the shaft 6 and cam housings 10, 14 are in place, the weather barrier 8 may be placed around the shaft 6. The weather barrier 8 will (see FIG. 9) typically be formed as two aluminum extruded halves 104, 106 having depressions 108 to accommodate the inner edges 110 of the door leaves 4, and may be fastened to the central shaft 6 in any desired way, e.g. by screws. The inner edges of the door leaves 4 may be provided with weather stripping 112 to seal the space between the leaves and the weather barrier.

After the shaft 6, upper and lower cam housings 10, 14, and weather barrier 8 have been installed, the door leaves 4 are placed in position. The leaves 4 are installed by sliding them onto the support arms 47, 47 so that these arms enter the slots 50, 50' near the upper and lower edges of the leaves. After the leaves 4 are in position, screws are inserted in apertures 114 through the leaf support arms, to hold the leaves in place.

The upper and lower edges of each leaf 4 are provided with channels 116 which hold conventional rubber sealing strips 118, to help seal the spaces between the leaves. The sealing strips 118 in the upper leaf channels extend up to the surface 100 of the ceiling 12, while the. rubber strips 118' in the lower leaf channels sweep the floor 16 and the ring 82.

It will be noted that the width of the upper and lower edges of each door leaf (typically about 1% inches) is greater than the width (typically about 1 inch) of the radial slots 30, 30' in the upper and lower cam housings. The leaves 4 (and the upper parts of the leaf support arms, with the blocks 52) therefore cover the slots 30, 30 when the leaves are in extended position, thereby protecting the cam housings against entry of dirt, and also hiding the slots from view, thus improving the appearance of the door.

The operation of the door will next be described, with reference to FIGURES to 13., which illustrate positions assumed by the leaves 4 and upper cam followers 36 as folding progresses. In FIGURES 10 to 13, the four door leaves, four cam followers, and four cam profile lobes and depressions are distinguished one from the other by the suffixes a, b, c and d.

Referring to FIGURE 10, assume that a panic has occurred and that people are streaming toward the door in the direction indicated by arrows A. When the people contact the leading two door leaves 4a, 4b, the pressure on these leaves is transmitted through the support arms 47, the pins 44a, 44b, and the cam followers 36a, 36b to the detent mechanisms holding these cam followers in position. (As discussed the detent mechanism for each cam follower is constituted by the spring 64 pressing the ball 62 into the groove 72 and at the same time pressing the root of the cam follower into the cam profile depression 26.)

When the pressure transmitted through leaves 4a, 4b exceeds the holding power of the detent mechanisms for cam followers 36a, 36b, the balls 62 of these cam followers are forced out of their grooves 72, and cam followers 36a, 36b rotate to the position shown in FIG. 11. As this pivoting occurs, the bottoms of the cam followers 36a, 36b in the upper cam housing 10 slide over the base plate 22, while the bottoms of the corresponding cam followers in the lower cam housing 14 slide over the cover plate 35. As soon as the outer ends of the cam followers move away from the circumferential wall 24, the balls 62 therein are urged to their outermost positions by their springs but cannot escape from the cam housings 36a, 36b because of the retaining caps 56a, 56b.

As indicated in FIGURE 11, the movement up to this point of the door leaves and cam housings has been a pure pivotal movement, with the leaf pins 44a, 44b serving as the pivotal axes. However, as the sides of the cam followers 36a, 36b strike the sides of the cam lobes 28, the leaf pins are forced to move radially outwardly in the slots 30a, 30b as shown in FIGURE 12. This produces a compound movement of the pins 44a, 44b, i.e. they undergo both translational and rotational movement. The door leaves 40, 411, being connected to pins 44a, 44b, move therewith and presently leaves 4a, 4b strike leaves 4c, 4d. The pressure on leaves 40, 4d causes their detent mechanisms to give way, and as movement of leaves 4a, 4b continues, the four leaves eventually assume the position shown in FIG. 13, in which the door is book folded.

In order to return the door leaves to their uncollapsed position, the leaves are simply grasped and moved around until the balls 62 are forced back into the grooves 72. In order to reduce the likelihood that the leaves will be moved too far (i.e. overshoot), the ball sockets 66 are placed in the apertures 32 such that their edges project about ,6 inch from the circumferential wall 24, as indicated by dimension d1 in FIG. 14, and their surfaces adjacent their grooves 72 project about inch from wall 24 (dimension d2 in FIG. 14). This causes the balls 62 to be depressed slightly before they enter grooves 72, providing a distinctive feel for an operator.

The doors may be manually folded simply by turning the handles 70 so that the grooves 72 are aligned horizontally. The balls 62 are now free to roll out of the grooves 72. It will be noted that no handles 70 are provided to turn the ball sockets 66' in the lower cam housing; instead, sockets 66' are fixed against rotation by set screws (not shown) inserted through the base plate of housing 14' and pressing on sockets 66'. In addition, sockets 66 contain ordinary partly spherical depressions (FIG. 15) for the balls 62, instead of grooves 72.

The sockets 66 are made non-rotatable for simplicity of operation. It is found that if the spring pressure of springs 64' in the lower cam followers is made relatively light, then rotation only of the upper sockets 66 will sufiice for easy manual folding of the doors. Typically the springs 64 in the lower cam followers 36' will exert a force about one-third to one-quarter that of the springs 64 in the upper cam followers 36.

It should be noted that although rotatable upper sockets 66 with grooves 72 and handles 70 may be provided if desired (e.g. to satisfy building codes), they are optional and will usually not be required. It is found that if the upper sockets 66 are made identical with lower sockets 66 (i.e. with rounded depressions 120 instead of grooves 70) and are fixed in position, and when springs 64, 64' are made of equal strength, then the door leaves may be broken readily when required, without requiring undue force.

It will be appreciated that various modifications may be made in the structure described. For example, four door leaves have been shown since this number is conventional, but this number may be reduced to three,

' with the number of slots in the cam housing also being reduced to three, and corresponding changes being made in the number of lobes and depressions in the cams and in the number of cam followers.

Further, it will be appreciated that various detent mechanisms may be used if desired to retain the door leaves in an extended position and to permit them to collapse upon application of a force greater than a predetermined force. The detent mechanism illustrated and described is believed to be a significant advance in the art, in view of its effectiveness and simplicity, but the use of other detent mechanisms would still leave remaining the feature that the door leaves are supported and guided by a relatively simple cam and cam follower mechanism of a type that this mechanism may be substantially entirely hidden in the floor and ceiling, thus making possible an uncluttered and modern appearance for the door. In addition, a detent mechanism may be provided only at the top (or bottom) of the door, although this will result in some loss in rigidity.

Further, the support arms 47 have been shown as largely buried inside the door leaves. They could instead be fastened to the upper and lower edges of the leaves, in full view, and could then be made with an attractive appearance. In this event the bodies of leaves can be made relatively thin if desired, but the support arms will have a width greater than that of slots 30, to hide these slots from view when the leaves are extended. Alternatively, the leaf support arms 47 can be entirely omitted, and instead the pins 44, 44' can project farther into the door leaves and can be clamped to the door leaves by clamping screws (similar to screws 49) at the inner ends of the leaves.

Moreover, changes may be made in the shape of the cam 18 and in the shapes of the cam followers 36, so long as the cam retains the roots of the cam followers when the leaves are extended, and so long as the cam assists in camming the cam followers to assist the cam follower roots and pins 44 to move radially outwardly along grooves 30 when pressure is exerted to collapse the door.

What I claim as my invention is:

1. For a collapsible revolving door having a central shaft, and door leaves normally rotating with said shaft, a mechanism for normally retaining said leaves in an extended position and for enabling folding of said leaves under predetermined conditions, said mechanism comprising:

(a) a cam housing adapted to be located adjacent one end of said shaft, said cam housing comprising (i) a central cam concentric with said shaft,

(ii) a base plate fixed to said cam and having slots therein one associated with each door leaf, each slot extending radially outwardly from said cam,

(b) a plurality of cam followers in said cam housing,

one associated with each door leaf, each cam follower having a root of a shape adapted to be held against pivotal movement by said cam when said root is held against said cam, and having a body extending from said root,

(c) detent means coupled between each cam follower and said cam housing for normally holding each cam follower in an extended position with the root of each cam follower held against said cam and the body of each cam follower extending radially outwardly from said cam over respective slots in said base plate,

(d) a pin fixed to each cam follower at a position adjacent the root thereof, each pin extending through that slot in said base plate over which its cam follower is located and having a tip adapted to be fixed to a door leaf to support and guide such leaf,

(e) and said detent means further including means yieldable upon application of a predetermined force transmitted thereto by pressure on a said door leaf acting through its associated pin and cam follower, to release such cam follower for movement inside said cam housing, whereby continued force on such door leaf will rotate its associated pin and cam followe in said cam housing until the body of such cam follower strikes said cam, further force on such door leaf then continuing to rotate such door leaf and cam follower and also forcing its associated pin radially outwardly in its slot in said base plate.

2. Apparatus according to claim 1, wherein said cam has a cam profile face substantially parallel with the axis of said shaft, said profile face having a depression associated with each door leaf, and a lobe between each depression, the roots of said cam followers being rounded to fit within said cam profile face depressions.

3. Apparatus according to claim 2 wherein said cam housing includes a circumferential wall fixed to said base plate concentric with said shaft, said cam followers extending substantially to said wall when in their extended position, and said detent means being coupled between said wall and the end of each cam follower facing said wall.

4. Apparatus according to claim 3 wherein said base plate provides a smooth bearing surface, and each cam follower has a smooth bearing surface slidable on said base plate, to facilitate movement of said cam followers in said cam housing upon folding of said door leaves.

5. Apparatus according to claim 4 wherein said detent means comprises: a spring in each cam follower, a ball retained in each cam follower and urged outwardly by said spring to a position in which the end of said ball projects from its cam follower, and depressions in said circumferential wall in which said balls are normally seated.

6. Apparatus according to claim 5 wherein said circumferential wall includes a plurality of rotatable ball sockets, said depressions in said wall being constituted by grooves in said sockets, said grooves normally being aligned vertically, said apparatus including means for rotating said sockets to realign said grooves horizontally to enable easy removal of said balls from said grooves to facilitate folding of said doors.

7. In a collapsible revolving door having a central vertical shaft, and door leaves normally in an extended position and rotating with said shaft, the improvement comprising:

(a) upper and lower cam housings, one adjacent the bottom and one adjacent the top of said shaft, each housing comprising:

(1) a central cam concentric with said shaft and fixed thereto, said cam having a substantially vertical cam profile face divided into a concave depression for each door segment, and a lobe between each depression,

(2) a base plate fixed to said cam and having slots therein one associated with each cam profile depression, each slot extending radially outwardly from a position adjacent its associated cam profile depression,

(3) and a substantially vertical circumferential wall fixed to said base plate and encircling said base plate adjacent the outer ends of said slots,

(b) a plurality of cam followers in each cam housing, one cam follower in each housing associated with each door leaf, each cam follower having a rounded root of a shape to fit within said cam profile depression, and having a body of width greater than the widths of said slots,

(c) detent means in each housing and coupled to each cam follower therein for normally holding each cam follower in an extended position with the root of each cam follower located in a cam profile face depression and its body extending radially outwlardly from such depression over a slot in said base p ate,

(d) a pin fixed to each cam follower adjacent the root thereof, each pin extending vertically through that slot in said base plate over which its carn follower is located,

(e) a support arm fixed to each pin at a position outside said cam housings, each support arm extending radially outwardly from said shaft,

(f) each leaf including means 'for connection with a support arm from said upper cam housing and an aligned support arm from said lower cam housing, to connect such leaf to such support arms for movement therewith,

(g) said leaves, when connected to and incorporating upper and lower support arms, having upper and lower edges of width greater than the width of said slots in said upper and lower cams and substantially covering said slots from view when in extended positron,

(h) and said detent means including means yieldable upon application of a predetermined force thereto caused by force on a door leaf acting through its associated upper and lower support arms, pins and cam followers coupled thereto, to release such cam followers for movement inside said cam housings whereby continued force on such door leaf will rotate its associated pins and cam followers inside said cam housings until the bodies of such cam followers strike said' lobes on said cams, continued force then moving such pins radially outwardly in their slots.

8. Apparatus according to claim 7 wherein said central shaft includes means for adjusting its length to facilitate installation of said door.

9. Apparatus according to claim 7 wherein said door leaves are four in number.

10. Apparatus according to claim 7 wherein said detent means includes a spring in each cam follower, a ball retained in each cam follower and urged outwardly therein by said spring to a position in which the end of said ball projects from its cam follower, and depressions in said circumferential wall into which Said balls are normally forced by said spring,

9 10 11. Apparatus according to claim 10 wherein the a horizontal direction to enable easy removal of said balls springs in the cam followers in said lower cam housing of the upper cam followers from said grooves to facilare of substantially lesser strength than the springs in itat ding 0f aid doors. the cam followers in said upper cam housing, and

wherein said circumferential Wall in said upper cam 5 References Cited housing includes a plurality of rotatable ball sockets, one UNITED STATES PATENTS for each ball, said depressions in said circumferential 2 04 717 7 193 Uhrich 49 44 wall being formed by grooves in said ball sockets, said 2 050 534 93 Peremi et 1 49 43 grooves normally being aligned in a vertical direction, 10 2 240 942 5 1941 Peremi et 1 49 44 said apparatus including means for rotating said ball sockets to a position in which said grooves are aligned in DENNIS L. TAYLOR, Primary Examiner 

